Method for manufacturing a rotary anode for X-ray tube

ABSTRACT

A method for manufacturing a rotary anode for use in an X-ray tube is proposed. The method has the step of forming, on a graphite substrate plate, an intermediate layer of rhenium by subjecting a metallic chloride to the thermal decomposition CVD process at a substrate temperature of 1200° C. or more. The method further has the step of forming, on the intermediate layer, an X-ray generating layer of tungsten or tungsten-rhenium alloy by subjecting a metallic fluoride to the hydrogen reduction thermal CVD process.

This is a divisional application of Ser. No. 07/882,026, filed May 13,1992.

BACKGROUND OF THE INVENTION

The present invention relates to a rotary anode for use in a rotaryanode type X-ray tube which is required to have a high X-ray output anda method for manufacturing the rotary anode.

With the increasing performance of medical equipment such as an X-rayCT, a more powerful X-ray output is required. Thus, a large heatcapacity as well as light weight is required for a rotary anode for usein an X-ray tube. In order to attain this purpose, it has been proposedto employ, in place of a rotary anode made of metal only, a structurecomprising a graphite substrate and an X-ray generating metal platebonded to the substrate or a graphite substrate plated with an X-raygenerating metal.

The former structure in which an X-ray generating metal plate is bondedto a graphite substrate is made by bonding an X-ray generating metalplate produced to the graphite substrate plate by brazing or the like(first prior art method). In the latter structure comprising a graphitesubstrate plated with an X-ray generating layer, the X-ray generatinglayer is formed by chemical vapor deposition on the graphite substrate,for example as disclosed in Japanese Patent publication 47-8263 (secondprior art method). The publication also teaches the formation of anintermediate layer of rhenium as a diffusion shield to prevent mutualdiffusion between tungsten, the best material as an X-ray generatinglayer, and the graphite substrate. Further, it is known to add rheniumto tungsten to prevent the X-ray generating intensity from dropping.Further, as disclosed in Japanese Unexamined Patent Publication 59-8252,it is known to subject the assembly to heat treatment in a non-oxidizingatmosphere after forming the intermediate layer and the X-ray generatinglayer by the second prior art method to form alloy layers between theadjacent layers and thus to improve adhesion between layers (third priorart method).

But the first prior art method has a problem in that the bonding of theX-ray generating metal plate is unstable. Further the metal plate has tobe made unnecessarily thick for structural reasons, so that the weightof the entire rotary anode cannot be markedly reduced.

The second prior art method is advantageous over the first prior artmethod in that a good adhesion is achieved between the graphitesubstrate and the X-ray generating layer, so that the thickness of theX-ray generating layer having a large specific gravity can be reduced toa minimum. But since the intermediate layer of rhenium has a columnargrain structure, mutual diffusion tends to occur between the tungstenand the graphite substrate. Such diffusion will cause the formation ofbrittle tungsten carbide under the X-ray generating layer. This may inturn cause reduced X-ray output or peeling or cracking of the X-raygenerating layer. Thus, this method is not suitable for prolonged use.

With the third prior art method, a tungsten carbide layer is formedunder the X-ray generating layer during the heat treatment step. Thus,this method cannot offer an effective solution to the problemsencountered by the second method.

SUMMARY OF THE INVENTION

The present invention provides a rotary anode for use in an X-ray tubeand a method for manufacturing the same, which obviates the aboveproblems. More specifically, a rotary anode for use in an X-ray tube isprovided comprising a graphite substrate plate, an intermediate layermade of a metal that does not react with graphite, and a generatinglayer provided on the intermediate layer for generating X-rays when anelectron impact is applied, characterized in that the intermediate layeris a rhenium film having an equiaxed grain structure. Also a method formanufacturing a rotary anode for use in an X-ray tube is provided,comprising the steps of forming, on a graphite substrate plate, anintermediate layer of rhenium by subjecting a metallic chloride to thethermal decomposition CVD process at a substrate temperature of 1200° C.or more, and forming, on the intermediate layer, a generating layer oftungsten or tungsten-rhenium alloy by subjecting a metallic fluoride tothe hydrogen reduction thermal CVD process.

First, we shall explain the reason why the rhenium film as theintermediate layer has to have an equiaxed grain structure. For purposesof this invention, the term "equiaxed grain structure" denotes an aspectratio of the grain nearly equal to one. The deposition temperatureduring the deposition process such as the CVD process is closelyassociated with the structure of the film obtained. This fact isreported in several publications. As shown in FIG. 3, a columnarstructure appears (zone 2) at comparatively low temperatures and anequiaxed grain structure appears (zone 3) at high temperatures. Thetemperature T₂ (in absolute temperature K) at which transition from thecolumnar structure to the equiaxed grain structure occurs is about halfof the melting temperature Tm (in absolute temperature K) of the metalused (T₂ / Tm=0.45 to be precise). If rhenium is used, since its meltingtemperature is 3180° C. (3453° K.), T₂ is about 1281° C. (1554° K.).

Actually, however, the equiaxed grain structure appeared at atemperature slightly lower than this temperature (1200° C. in thepresent invention). It was observed that carbon diffuses through arhenium film having an equiaxed grain structure at a slower speed thanthrough a rhenium film having a columnar structure. This is due to thefact that the diffusion of carbon through the grain boundary of acolumnar structure (grain boundary diffusion) is faster than thediffusion of carbon inside the crystal grain (bulk diffusion), i.e. dueto the fact that the diffusing distance through the grain boundary of anequiaxed grain structure is longer than that through the grain boundaryof a columnar structure. Also, even if two materials have the samecolumnar structure, the smaller the grain diameter in the columnarstructure, the greater The number of diffusion paths and thus thegreater amount of carbon moves by diffusion.

On the other hand, it is known that the life of a rotary anode having anX-ray generating layer containing tungsten as a main substance on thegraphite substrate can shorten if the carbon that diffuses through therhenium intermediate layer reacts with the tungsten to form tungstencarbide and this causes peelings and cracking in the X-ray generatinglayer. Therefore, a rotary anode for use in an X-ray tube whichwithstands long use can be obtained if a rhenium film having an equiaxedgrain structure is formed as the intermediate layer or diffusion shieldto restrain carbon diffusion according to the present invention.

Next, as a method of forming the rotary anode according to the presentinvention, the intermediate layer is formed from a rhenium chloride bythe thermal decomposition CVD. This is to achieve the following merits.

First, the thermal decomposition reaction offers higher activationenergy than the hydrogen reduction method. Thus, the reaction occurs athigher temperature and a finer film is obtainable. For example, afluoride gas was frequently used in the hydrogen reduction methodbecause of relative easiness in handling a material gas. But with thismethod, since the reaction starts at an extremely low temperature, about300° C., the density of the film obtained tends to be low In contrast bysubjecting a chloride to thermal decomposition, the film density neverdrops even if the board temperature is increased to 1200° C. or more.

The second merit is that the rhenium film thus made has larger crystalgrains and an equiaxed grain structure. We have already explained whyand how a rhenium film having an equiaxed grain structure has excellentproperties as a diffusion shield.

The third merit is a low production cost. A rhenium film can be formedfrom a rhenium fluoride. But a rhenium fluoride is very expensive.According to the present invention, a relatively inexpensive rheniumchloride is used. For example, as described in Japanese UnexaminedPatent Publication 58-100669, by synthesizing (chlorinating) rheniumchloride from relatively inexpensive materials, that is, metallicrhenium and chlorine gas in a chlorinating oven provided upstream of theCVD reactor and then decomposing it in the CVD reactor provideddownstream thereof, the yield of the material improves greatly.

The X-ray generating layer is formed from a metallic fluoride byhydrogen reduction. This is because this method is suitable for forminga film made up of fine crystal grains. Since the tungsten as the mainsubstance in the generating layer is heated to 1000° C. or more byelectron impact, it recrystallizes gradually and its ductility drops. Itis known that such drop in ductility can be prevented by adding several% to 30% of rhenium to tungsten. It is also possible to further restrainthe drop in ductility by forming a film structure which is difficult torecrystallize. The film structure which is difficult to recrystallize isa structure made up of fine crystal grains. To form such a film, thehydrogen reduction method showed the best results.

For lower production cost, the intermediate layer and the generatinglayer are preferably formed continuously in the same reactor. Accordingto the present invention, a reactor is used which is equipped with botha chloride generating device (that is, a chlorinating oven) and afluoride supply line to allow a continuous coating.

Preferably, the rhenium intermediate layer having an equiaxed structureshould have an average grain size in the range of 10-50 microns toachieve optimum results. Similarly, the thickness of this intermediatelayer should be about 10-50 microns to achieve the best results.

As described above, according to the present invention, a rhenium filmhaving an equiaxed grain structure is used as the diffusion shield.Thus, the mutual diffusion between the tungsten and the graphitesubstrate is restrained and the X-ray generating intensity can be kepthigh for a prolonged period of time. Also, the method according to thepresent invention is suitable for manufacturing the above-describedrotary anode with low manufacturing cost. Thus a high-output X-raysource can be manufactured which is useful for improving the accuracy ofindustrial X-ray analyzers as well as medical ones.

Other features and objects of the present invention will become apparentfrom the following description taken with reference to the accompanyingdrawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of the rotary anode according to the presentinvention showing how the substrate is coated with the X-ray generatinglayer and the intermediate layer;

FIG. 2 is a scanning electron microscopic photograph of the rotary anodeaccording to the present invention showing how the substrate is coatedwith the X-ray generating layer and the intermediate layer; and

FIG. 3 is an explanatory view showing the relation between thedeposition temperature and the film obtained,

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT EXAMPLE 1

We examined the structures of the rhenium films obtained by subjecting arhenium chloride to the thermal decomposition CVD process underdifferent rhenium film forming conditions. In this test, films wereformed to a thickness of 30 microns with the gas conditions constant andthe temperature varied around the substrate temperature at the time ofdeposition. Also, as comparative examples, we formed rhenium films bysubjecting metallic chlorides and metallic fluorides to hydrogenreduction. The sectional structures were evaluated by observing thesections of the coated specimens through a scanning electron microscope(FIG. 2 shows a microscopic photo). The results are shown in Table 1.

As is apparent from Table 1, with the thermal decomposition CVD processfor rhenium chloride, a fine film having an equiaxed grain structure wasobtained at 1200° C. or more. On the other hand, with the hydrogenreduction of chloride, a powdery deposit was obtained at 1000° C. ormore, while a rhenium film having a columnar structure was obtained at1000° C. or less. With the method in which fluorides are subjected tohydrogen reduction, a powdery deposit was obtained at 700° C. or morewhile a rhenium film having a columnar structure was obtained at 700° C.or less, It was thus confirmed that the thermal decomposition CVDprocess for chloride is suitable for forming a rhenium film having anequiaxed grain structure at a temperature of 1200° C. or more.

EXAMPLE 2

A 500-micron thick tungsten coating was formed by subjecting metallicfluorides to hydrogen reduction on each of the rhenium coated specimensobtained in the Example 1. After annealing at a constant temperature of1200° C. for 10 hours, they were examined to see how carbide layer grewin the tungsten film. The results are shown in Table 2.

As is apparent from Table 2, comparing those having columnar structures,the larger the crystal grains, the more slowly the tungsten carbidelayer grows. Also. it is apparent that the tungsten carbide layer growsfurther slowly in a film having an equiaxed grain structure comparedwith a film having a columnar structure. Thus, it was confirmed that arhenium film having an equiaxed grain structure is superior as thediffusion shield.

                  TABLE 1                                                         ______________________________________                                                           depo-              crystal                                                    sition             grain                                   material                                                                             reaction    temp.   film       diameter                                gas    method      (°C.)                                                                          structure  (micron)                                ______________________________________                                        chloride                                                                             thermal      800    columnar struc-                                                                          5                                              decomposition       ture                                                      process     1000    columnar struc-                                                                          8                                                                  ture                                                                  1200    equiaxed grain                                                                           12                                                                 structure                                                             1400    equiaxed grain                                                                           16                                                                 structure                                          chloride                                                                             hydrogen     800    columnar struc-                                                                          4                                              reduction           ture                                                      process     1000    powdery deposit                                                                          --                                                         1200    powdery deposit                                                                          --                                      fluoride                                                                             hydrogen     500    columnar struc-                                                                          2                                              reduction           ture                                                      process      700    powdery deposit                                                                          --                                                          900    powdery deposit                                                                          --                                      ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                                                       crystal                                                                              carbide                                                                grain  layer                                   material                                                                             reaction    film        diameter                                                                             thickness                               gas    method      structure   (micron)                                                                             (micron)                                ______________________________________                                        fluoride                                                                             hydrogen    columnar grain                                                                            2      18                                             reduction   structure                                                  chloride                                                                             process     columnar grain                                                                            4      10                                                         structure                                                  chloride                                                                             thermal     columnar grain                                                                            5      9                                              decomposition                                                                             structure                                                         process     columnar grain                                                                            8      5                                                          structure                                                                     equiaxed grain                                                                            12     3                                                          structure                                                                     equiaxed grain                                                                            16     2                                                          structure                                                  ______________________________________                                    

What is claimed is:
 1. A method for manufacturing a rotary anode for usein an X-ray tube, said method comprising the steps of forming, on agraphite substrate plate, an intermediate layer of rhenium by subjectingrhenium chloride to a thermal decomposition CVD process at a substratetemperature of 1200° C. or more, and forming, on said intermediatelayer, an X-ray generating layer of tungsten or tungsten-rhenium alloyby subjecting tungsten fluoride or both tungsten fluoride and rheniumfluoride to a hydrogen reduction thermal CVD process; wherein saidintermediate layer is about 10-50 microns thick and consists essentiallyof rhenium having an equiaxed grain structure which is free fromcolumnar grain structure, and an average grain size of about 10-50microns.